Association of BRCA1 with the Inactive X Chromosome and XIST RNA
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FirstCite Published online e-publishing Association of BRCA1 with the inactive X chromosome and XIST RNA Shridar Ganesan1, Daniel P. Silver1, Ronny Drapkin1, Roger Greenberg1, Jean Feunteun2 and David M. Livingston1* 1The Dana-Farber Cancer Institute and Harvard Medical School, 1 Jimmy Fund Way, Boston, MA 02115, USA 2Laboratoire de Genetique et Transfert de Gene, Institut Gustav-Roussy, Villejuif 94805, France Breast cancer, early onset 1 (BRCA1) encodes a nuclear protein that participates in breast and ovarian cancer suppression. The molecular basis for the gender and tissue specificity of the BRCA1 cancer syn- drome is unknown. Recently, we observed that a fraction of BRCA1 in female cells is localized on the inactive X chromosome (Xi). Chromatin immunoprecipitation (ChIP) experiments have demonstrated that BRCA1 physically associates with Xi-specific transcript (XIST) RNA, a non-coding RNA known to coat Xi and to participate in the initiation of its inactivation during early embryogenesis. Cells lacking wild- type BRCA1 show abnormalities in Xi, including lack of proper XIST RNA localization. Reintroduction of wild-type, but not mutant, BRCA1 can correct this defect in XIST localization in these cells. Depletion of BRCA1 in female diploid cells led to a defect in proper XIST localization on Xi and in the development of normal Xi heterchromatic superstructure. Moreover, depletion of BRCA1 led to an increased likelihood of re-expression of a green fluorescent protein (GFP) reporter gene embedded on Xi. Taken together, these findings are consistent with a model in which BRCA1 function contributes to the maintenance of proper Xi heterochromatin superstructure. Although the data imply a novel gender-specific consequence of BRCA1 loss, the relevance of the BRCA1/Xi function to the tumour suppressor activity of BRCA1 remains unclear and needs to be tested. Keywords: BRCA1; XIST; X chromosome inactivation; heterochromatin; breast cancer 1. INTRODUCTION BRCA1 (Gowen et al. 1998; Moynahan et al. 1999; Scully et al. 1999). The BRCA1 gene encodes an 1863 residue nuclear pro- Overexpression of BRCA1 can alter expression of a var- tein that has little homology to other known proteins. iety of genes including GADD45 (growth arrest and DNA Women carrying a germline mutation in BRCA1 experi- damage-inducible 45) and may modulate ligand and ence a greatly increased lifetime risk of breast and ovarian ligand-independent activation of the oestrogen receptor cancer. The tumours that arise in these individuals all (Harkin et al. 1999; Zheng et al. 2000; Fan et al. 2001). reveal loss of the wild-type and retention of the mutant Recent evidence suggests that BRCA1 may have a role allele of BRCA1, implying that this gene functions as a in the regulation of chromatin structure, as seen by its tumour suppressor. Interestingly, men who harbour a association with the SWI/SNF (mating type switch/ BRCA1 mutation do not have a clear increase in cancer sucrose non fermenting) complex and by reports that predisposition (Thompson & Easton 2002). overexpression of BRCA1 can alter the structure of certain BRCA1 is a nuclear protein found in all proliferating chromatin domains (Bochar et al. 2000; Ye et al. 2001). cells that have been tested and whose expression appears BRCA1, together with its partner protein, BARD1, also to be cell-cycle regulated. Extensive investigation has exhibits in vitro E3 ubiquitin ligase activity (Hashizume shown that BRCA1 probably plays a role in several funda- et al. 2001). How these potential functions are related to mental cellular processes including the maintenance of BRCA1-tumour suppression, and how they contribute to genomic integrity, transcriptional regulation and cell-cycle the gender and tissue specificity of the BRCA1 cancer syn- checkpoint regulation (Deng & Brodie 2000; Scully & Liv- drome in humans remain unclear. ingston 2000; Venkitaraman 2002). Cells lacking wild- type BRCA1 rapidly develop chromosomal abnormalities and experience abnormal DNA damage repair and altered 2. BRCA1 LOCALIZES TO THE INACTIVE cell-cycle checkpoint regulation. All of these defects can X-CHROMOSOME IN SPERMATOCYTES be partly complemented by reintroduction of wild-type AND FEMALE SOMATIC CELLS BRCA1 mRNA and protein are highly expressed in the testis. In zygotene spermatocytes, BRCA1 is detected on * Author for correspondence ([email protected]). the unsynapsed arms of developing synaptanemal com- One contribution of 18 to a Discussion Meeting Issue ‘Replicating and plexes together with RAD51 (Scully et al. 1997). In the reshaping DNA: a celebration of the jubilee of the double helix’. more abundant pachytene spermatocytes, BRCA1 heavily Phil. Trans. R. Soc. Lond. B 03TB040N.1 2003 The Royal Society DOI 10.1098/rstb.2003.1371 03TB040N.2 S. Ganesan and others BRCA1 association with Xi and XIST RNA (a)(b)(c) (d)(e)( f ) (g)(h) BARD1 BRCA1 RNA IgG BARD1 BRCA1 RNA RT: – + – + – + – + + + + XIST H19 (ex 6) BARD1 BRCA1 RNA Figure 1. Association of BRCA1 and markers of Xi (reproduced from Ganesan et al. (2002)1 with permission). (a–f ) Combined immunostaining of telomerase immortalized human mammary epithelial cells (a–c) and WI-38 female human fibroblasts (d–f ) with a monoclonal antibody to BRCA1 (a,d), and RNA FISH for XIST (b,e). Merged images are shown in the right panels (c,f ). (g) ChIP was performed on 293 cells by using purified antibodies to BRCA1, BARD1 and an irrelevant antibody. RNA was extracted from each ChIP and subjected to RT-PCR using primers specific for a region in exon 6 of XIST. The (ϩ) and (Ϫ) signify PCR amplification with and without first round RT. The last two lanes are controls, using cellular RNA as a template. (h) ChIPs, performed on 293 cell extract, were subjected to RT-PCR using primers specific for H19 RNA. decorates the unpaired X chromosome (Scully et al. 1997; BRCA1 with markers of Xi, such as MH2A1 and XIST Ganesan et al. 2002). In pachytene cells, the unpaired X RNA on a nuclear structure (figure 1). This colocalization and Y chromosomes undergo dramatic changes in was apparent in only a small fraction of cells, but it was chromatin structure, becoming densely heterochromatic, present in multiple female cell lines and strains and transcriptionally silenced, and localized at the nuclear per- appears to be cell-cycle dependent (Ganesan et al. 2002). iphery in a discrete structure called the XY body (Handel & Hunt 1992). The pachytene cell X chromo- some accumulates the non-coding XIST RNA, becomes enriched with the histone variant MH2A1, and undergoes 3. ASSOCIATION OF BRCA1 AND BARD1 WITH methylation of histone H3 on lysine 9 (Richler et al. 1992, XIST RNA 2000; Ayoub et al. 1997; Costanzi & Pehrson 1998). These changes are remarkably similar to the changes that ChIP performed with antibodies specific to BRCA1 occur on Xi in female somatic cells (Clemson et al. 1996; demonstrated the presence of XIST RNA. (figure 1). No Jaenisch et al. 1998; Avner & Heard 2001; Huynh & Lee such band was detected when irrelevant antibody was 2001), and led us to investigate whether BRCA1 func- used. Moreover, ChIP, performed with an antibody to tionally interacts with Xi. BARD1, a structurally related protein that efficiently het- BRCA1 immunostaining appears in proliferating cells erodimerizes with BRCA1, also co-immunoprecipitated as multiple nuclear foci that are most apparent during S XIST RNA (figure 1). By contrast, the same ChIPs lacked and G2 phases of the cell cycle. In a variety of human H19 RNA, another non-coding RNA (Ganesan et al. female (but not male) cell lines and strains, a subset of 2002). These findings suggest that BRCA1 and BARD unsynchronized cells demonstrated colocalization of interact, directly or indirectly, with XIST RNA. Phil. Trans. R. Soc. Lond. B BRCA1 association with Xi and XIST RNA S. Ganesan and others 03TB040N.3 (a)(b) (c)(d)(e) ( f )(g)(h) Figure 2. XIST RNA FISH analysis of naive and BRCA1-reconstituted HCC1937 cells (reproduced from Ganesan et al. (2002)1 with permission). (a,b) XIST RNA FISH staining of vector-reconstituted HCC (a) or HCC that were stably reconstituted with wild-type BRCA1 (b). (c–e) HCC stably transfected with a doxycycline inducible wild-type BRCA1 vector (HCC clone 5) were exposed to doxycycline for 24 h and subjected to combined immunostaining for BRCA1 (d) and RNA FISH for XIST (e). Nuclei were stained with 4Ј, 6-diamidino-2-phenylindole (DAPI) (c). The arrow points to the region of greatest BRCA1 staining intensity, which coincides with the region that stains for XIST RNA. ( f–h) Simultaneous DNA FISH for an X-chromosome probe ( f ) and RNA FISH for XIST (g) were performed on HCC clone 5 cells stimulated with doxycyclin. A merged image is shown in (h). 4. BRCA1-DEFICIENT TUMOUR CELLS REVEAL single focus of XIST staining by RNA FISH (figure 2). ABNORMALITIES IN XIST LOCALIZATION Similar reconstitution with disease-associated, point missense-mutants of BRCA1 failed to induce focal XIST To determine whether BRCA1 maintains a functional staining, suggesting that intact BRCA1 is required for this interaction with Xi, certain characteristic features of Xi effect. Naive and wild-type BRCA1-reconstituted were examined in tumour cells lacking wild-type BRCA1. HCC1937 cells, although characterized by clear differ- HCC1937 cells are human breast cancer cells that lack ences in XIST staining by RNA FISH, did not reveal wild-type BRCA1. Most HCC1937 cells contain two X- measurable differences in XIST RNA levels as seen by chromosomes, although the line is aneuploid, and certain quantitative RT-PCR. This observation suggests that cells contain greater than two X chromosomes (Ganesan BRCA1 does not affect XIST RNA synthesis or stability, et al. 2002). Despite having at least two X-chromosomes, but, by exclusion, may support proper localization of this cell line lacks several normal features of Xi, including XIST RNA on Xi.